ruby-c2pa

A Ruby gem for signing and reading C2PA content credentials in media files. Built on top of the official c2pa-rs Rust library via a native extension.

What is C2PA?

C2PA (Coalition for Content Provenance and Authenticity) is an open technical standard for attaching cryptographically signed provenance metadata to media files. It lets you prove:

Who created or edited a file
What tools were used
When and where it was created
Whether the content has been tampered with since signing

It is backed by Adobe, Microsoft, Google, the BBC, and others, and is increasingly required by platforms and publishers to establish trust in digital media — particularly in an era of AI-generated content.

Why Rust bindings?

The C2PA specification is complex and security-sensitive. The reference implementation is c2pa-rs, an official Rust library maintained by the Content Authenticity Initiative. Rather than re-implementing the specification in Ruby (which would risk diverging from the spec or introducing security bugs), this gem wraps c2pa-rs directly.

The binding layer is a native Ruby extension written in Rust using magnus, which compiles directly into a .bundle/.so that Ruby loads like any other native extension. This means:

Correctness — you get the reference implementation, not a reimplementation
Security — cryptographic signing and manifest validation are handled by audited Rust code
Performance — signing large video files happens in native code with no Ruby overhead
Spec compliance — as c2pa-rs is updated to track the spec, you get those updates by bumping the Rust dependency

Requirements

Ruby >= 3.0
Rust and Cargo (to compile the native library)
OpenSSL (usually already present on macOS and Linux)

Installing Rust

The compilation happens automatically during gem install, but Rust must be present on your system first.

The recommended way is via mise, which can manage both Ruby and Rust in one place:

mise use --global rust@latest

Or via the official rustup installer:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Installation

Add to your Gemfile:

gem "ruby-c2pa"

Then run:

bundle install

The native Rust library is compiled automatically during installation. This takes a few minutes the first time as it downloads and compiles the c2pa-rs dependency tree.

Preparing your certificate and key

C2PA signing requires an X.509 certificate chain and private key in PEM format. The certificate file contains the end-entity certificate first, then any intermediates, and must not include the root.

c2pa-rs enforces a certificate profile. An end-entity certificate is rejected unless it carries all of:

Basic Constraints CA:FALSE, critical
Key Usage with digitalSignature or nonRepudiation, critical
an Extended Key Usage that is present and not any, critical
a Subject Key Identifier
an Authority Key Identifier

The last one is easy to miss — openssl x509 -req omits it by default, and the certificate is then rejected with nothing more specific than the certificate is invalid.

Whose CA?

A certificate from a CA in the C2PA trust list validates as Trusted with no configuration.

A certificate from your own CA validates as Valid and carries signingCredential.untrusted — which does not make the manifest invalid. To have it treated as trusted, add your root as a trust anchor; see Configuring trust.

Development and testing

The test suite generates its own certificates for every supported algorithm:

bundle exec rake fixtures:certs

They land in test/fixtures/certs/ and are not committed. See test/fixtures/generate_certs.rb for a worked example of building a chain c2pa-rs accepts.

The supported signing algorithms are: es256, es384, es512, ps256, ps384, ps512, ed25519.

Usage

Building a manifest

Every signed file requires a C2PA::Manifest with at least one action. Actions describe what happened to the asset and are drawn from the C2PA::Actions constants, which cover the full vocabulary defined in the C2PA specification.

require "c2pa"

manifest = C2PA::Manifest.new(title: "Sunset over the bay")
manifest.add_action(
  C2PA::Actions::CREATED,
  digital_source_type: C2PA::DigitalSourceTypes::DIGITAL_CAPTURE
)

c2pa.created must declare how the asset came into being. c2pa-rs rejects a manifest without it, and accepts any string you supply — so a wrong value validates while asserting something untrue. The gem therefore requires you to choose rather than defaulting on your behalf:

Constant	Use for
`DIGITAL_CAPTURE`	a camera original
`TRAINED_ALGORITHMIC_MEDIA`	generative AI output
`COMPOSITE_WITH_TRAINED_ALGORITHMIC_MEDIA`	edited using generative AI
`SCREEN_CAPTURE`	a screenshot
`HUMAN_EDITS`	human-edited media
`UNSPECIFIED`	the origin is genuinely unknown

C2PA::DigitalSourceTypes::ALL lists them all. Reach for UNSPECIFIED when you do not know — it is what c2pa-rs uses in its own fixtures, and it is honest in a way that guessing is not.

This requirement arrived in c2pa-rs 0.90. Manifests signed by releases before 0.3.0 omit the field and are rejected by current verifiers.

Actions can be chained:

manifest = C2PA::Manifest.new(title: "Sunset over the bay")
  .add_action(C2PA::Actions::CREATED,
              digital_source_type: C2PA::DigitalSourceTypes::DIGITAL_CAPTURE)
  .add_action(C2PA::Actions::PUBLISHED)

Editing an existing asset

When the file you are signing derives from another one, declare the intent rather than adding c2pa.opened yourself:

manifest = C2PA::Manifest.new(title: "Edited photo", intent: :edit)
  .add_action(C2PA::Actions::EDITED)
  .add_action(C2PA::Actions::PUBLISHED)

c2pa-rs derives the parent ingredient from the source file and adds a c2pa.opened action tied to it, so the signed manifest records c2pa.opened, c2pa.edited, c2pa.published and a parentOf ingredient.

c2pa.opened cannot be added by hand. The specification requires it to reference its parent ingredient by hashed URI, and that hash is computed over the ingredient as c2pa-rs serialises it — so add_action(C2PA::Actions::OPENED) raises and points here. Earlier releases of this gem documented adding it directly; manifests built that way never validated.

Each action accepts optional fields from the C2PA specification:

manifest.add_action(
  C2PA::Actions::CREATED,
  when_time:           "2026-03-17T10:00:00Z",   # ISO 8601 timestamp
  software_agent:      "Acme Editor/2.0",          # defaults to "ruby-c2pa/<version>"
  digital_source_type: "https://cv.iptc.org/newscodes/digitalsourcetype/trainedAlgorithmicMedia",
  changed:             ["region_of_interest"],
  parameters:          { "description" => "Generated by AI" }
)

Available actions

All actions defined in the C2PA specification are available as constants on C2PA::Actions:

Constant	Value
`C2PA::Actions::CREATED`	`c2pa.created`
`C2PA::Actions::OPENED`	`c2pa.opened`
`C2PA::Actions::EDITED`	`c2pa.edited`
`C2PA::Actions::EDITED_METADATA`	`c2pa.edited.metadata`
`C2PA::Actions::ADJUSTED_COLOR`	`c2pa.adjustedColor`
`C2PA::Actions::CHANGED_SPEED`	`c2pa.changedSpeed`
`C2PA::Actions::CONVERTED`	`c2pa.converted`
`C2PA::Actions::CROPPED`	`c2pa.cropped`
`C2PA::Actions::DELETED`	`c2pa.deleted`
`C2PA::Actions::DRAWING`	`c2pa.drawing`
`C2PA::Actions::DUBBED`	`c2pa.dubbed`
`C2PA::Actions::ENHANCED`	`c2pa.enhanced`
`C2PA::Actions::FILTERED`	`c2pa.filtered`
`C2PA::Actions::ORIENTATION`	`c2pa.orientation`
`C2PA::Actions::PLACED`	`c2pa.placed`
`C2PA::Actions::PUBLISHED`	`c2pa.published`
`C2PA::Actions::REDACTED`	`c2pa.redacted`
`C2PA::Actions::REMOVED`	`c2pa.removed`
`C2PA::Actions::REPACKAGED`	`c2pa.repackaged`
`C2PA::Actions::RESIZED`	`c2pa.resized`
`C2PA::Actions::TRANSLATED`	`c2pa.translated`
`C2PA::Actions::TRANSCODED`	`c2pa.transcoded`
`C2PA::Actions::TRIMMED`	`c2pa.trimmed`
`C2PA::Actions::UNKNOWN`	`c2pa.unknown`
`C2PA::Actions::WATERMARKED`	`c2pa.watermarked`

Adding other assertions

Use add_assertion for any assertion type beyond actions, such as schema.org metadata or AI training preferences:

manifest.add_assertion(
  label: "stds.schema-org.CreativeWork",
  data: {
    "@context" => "https://schema.org",
    "@type"    => "CreativeWork",
    "author"   => [{ "@type" => "Person", "name" => "Jane Smith" }]
  }
)

Adding ingredients

Ingredients record the source assets a file was derived from. Supply the file so c2pa-rs can read it:

manifest.add_ingredient(
  title:        "Original photo",
  format:       "image/jpeg",
  instance_id:  "xmp:iid:original-uuid-here",
  relationship: "componentOf",
  file:         "original.jpg"
)

If the ingredient already carries content credentials, its manifest is embedded in the signed output and the ingredient points at it. A verifier can then follow the chain from your asset back through the original. For a file with no credentials there is nothing to carry forward.

Omitting file: records the description alone. Nothing binds it to any bytes, so a verifier cannot check the claim. That form is kept for compatibility; prefer the file.

Updating an existing asset

For a non-editorial change to an asset, such as correcting metadata, use intent: :update. The source file is the parent, and the change is recorded against it without opening a new editing lineage:

manifest = C2PA::Manifest.new(title: "Metadata corrected", intent: :update)
  .add_action(C2PA::Actions::EDITED_METADATA)

c2pa-rs restricts this mode: there is exactly one ingredient, it is the source itself, and the hashed content must not change.

Signing a file

The output path must not already exist — C2PA.sign will raise a C2PA::SigningError if the file is already there.

manifest = C2PA::Manifest.new(title: "Sunset over the bay")
  .add_action(C2PA::Actions::CREATED,
              digital_source_type: C2PA::DigitalSourceTypes::DIGITAL_CAPTURE)

C2PA.sign(
  file:        "photo.jpg",
  output:      "photo_signed.jpg",
  certificate: "test_cert.pem",
  key:         "test_key.pem",
  manifest:    manifest
)

C2PA.sign reads the signed file back and confirms it validates before returning. If it does not, the output file is deleted and a C2PA::SigningError is raised naming the failure codes.

This matters because c2pa-rs applies its rules when reading, not when writing. Signing reports success for manifests that every verifier rejects, which is exactly what earlier versions of this gem did — silently, for months. The check costs one extra read of the output.

Turn it off with verify: false if you want the file kept for inspection:

C2PA.sign(
  file:        "photo.jpg",
  output:      "photo_signed.jpg",
  certificate: "cert.pem",
  key:         "key.pem",
  manifest:    manifest,
  verify:      false
)

Specify a different signing algorithm with algorithm: (default is "es256"):

C2PA.sign(
  file:        "photo.jpg",
  output:      "photo_signed.jpg",
  certificate: "cert.pem",
  key:         "key.pem",
  algorithm:   "ps256",
  manifest:    manifest
)

Signing bytes in memory

For data that never touches the filesystem, such as an upload held in a request body or an image your application generated, C2PA.sign_buffer takes the bytes and returns the signed bytes. The format must be given, since there is no filename to infer it from.

signed = C2PA.sign_buffer(
  data:        request.body.read,
  format:      "image/jpeg",
  certificate: "cert.pem",
  key:         "key.pem",
  manifest:    manifest
)

The input must be a binary string (Encoding::BINARY, which is what File.binread and IO#read on a binary-mode stream return). A string tagged UTF-8 is rejected with an ArgumentError rather than transcoded, because a transcoded JPEG is a corrupt JPEG and nothing notices until a verifier rejects it. If you have such a string and know the bytes are intact, call .b on it.

The same verify-after-sign guard applies. A result that does not validate is never returned; C2PA::SigningError is raised instead, and verify: false returns it anyway. algorithm: and everything on the manifest, including intents and ingredient files, work as they do for C2PA.sign.

Memory is the trade-off. The input, the copy c2pa-rs works on, and the signed result on both sides of the Ruby boundary are resident at once at the peak, so budget about four times the size of the asset per call. For a photo that is nothing; for a feature-length video it is a reason to use C2PA.sign with paths instead.

Signing and reading release Ruby's global VM lock while c2pa-rs works, so other threads in the process keep running. A threaded server signing a large video does not stall its other requests for the duration. One consequence: Thread#kill and Timeout cannot interrupt a native call in progress; they take effect when it returns.

Reading a manifest

result = C2PA.read(file: "photo_signed.jpg")

active = result["manifests"][result["active_manifest"]]
puts active["title"]
puts active["claim_generator_info"].first["name"]   # => "ruby-c2pa"

From memory, C2PA.read_buffer takes the bytes. c2pa-rs identifies most formats from the leading bytes, so the format is optional; it is needed for a format with no signature to sniff, such as SVG.

result = C2PA.read_buffer(data: signed)
result = C2PA.read_buffer(data: svg_bytes, format: "image/svg+xml")

Naming your application

Signed files credit ruby-c2pa by default. To credit your own application instead:

manifest = C2PA::Manifest.new(
  title:             "Sunset over the bay",
  generator_name:    "Acme Editor",
  generator_version: "2.0"
).add_action(
  C2PA::Actions::CREATED,
  digital_source_type: C2PA::DigitalSourceTypes::DIGITAL_CAPTURE
)

The signed manifest then reads:

{
  "name": "Acme Editor",
  "version": "2.0",
  "org.contentauth.c2pa_rs": "0.90.22",
  "org.rubygems.ruby_c2pa": "0.5.0"
}

c2pa-rs permits exactly one claim generator entry, so your application replaces the gem as the name rather than preceding it. The gem is recorded in a namespaced field alongside it, which is how c2pa-rs records itself.

Releases before 0.3.0 credited c2pa-rs and named neither the gem nor the calling application.

Checking the SDK version

puts C2PA.sdk_version  # => "0.90.22"  (the c2pa-rs version the gem was built against)

Error handling

All errors inherit from C2PA::Error, so you can rescue broadly or narrowly. SigningError covers signing and the post-signing verification, ReadError reading, InvalidManifestError anything the builder rejects, and InvalidSettingsError anything C2PA.configure cannot use.

begin
  C2PA.sign(file: "photo.jpg", output: "photo_signed.jpg", certificate: "cert.pem", key: "key.pem", manifest: manifest)
rescue C2PA::InvalidManifestError => e
  puts "Manifest is invalid: #{e.message}"
rescue C2PA::SigningError => e
  puts "Signing failed: #{e.message}"
end

begin
  C2PA.read(file: "photo_signed.jpg")
rescue C2PA::ReadError => e
  puts "Could not read manifest: #{e.message}"
end

begin
  C2PA.configure { |config| config.trust_anchors = "ca/root.pem" }
rescue C2PA::InvalidSettingsError => e
  puts "Settings not usable: #{e.message}"
end

# Or rescue any C2PA error broadly
begin
  C2PA.sign(file: "photo.jpg", output: "photo_signed.jpg", certificate: "cert.pem", key: "key.pem", manifest: manifest)
rescue C2PA::Error => e
  puts "C2PA error: #{e.message}"
end

Configuring trust

c2pa-rs checks the signing certificate against a trust list, and reports Trusted when it chains to a root that list contains. That happens by default, so a certificate from a CA in the C2PA trust list needs no configuration.

For a private or enterprise CA, add its root:

C2PA.configure do |config|
  config.trust_anchors = "ca/root.pem"   # a path, or the PEM text itself
end

Files signed by a certificate chaining to it then validate as Trusted rather than carrying signingCredential.untrusted.

Offline and air-gapped environments

Reading an asset may fetch a remote manifest over the network, and revocation checking may contact an OCSP responder. Both can be turned off:

C2PA.configure do |config|
  config.remote_manifest_fetch = false
  config.ocsp_fetch            = false
end

Thumbnails

c2pa-rs can embed a thumbnail of the asset in its manifest, and of each ingredient supplied as a file. Verify tools show it alongside the credentials. It is off unless you turn it on:

C2PA.configure do |config|
  config.thumbnails     = true
  config.thumbnail_size = 512      # longest edge in pixels
end

The default is off because c2pa-rs scales to a fixed long edge and upscales to reach it. Its own default is 1024, so a 160×120 image gets a 1024×768 thumbnail, roughly ten times the size of the asset it describes. Set thumbnail_size no larger than your assets, or leave thumbnails off for small images.

Thumbnails are produced for JPEG, PNG, WebP and TIFF. Other formats sign without one; c2pa-rs treats that as non-fatal.

Everything configurable

Setting	Default	Purpose
`trust_anchors`	none	additional roots to trust, as PEM
`trust_list`	C2PA list	replaces the trust list rather than adding to it
`allowed_certificates`	none	explicitly allowed certificates, as PEM
`verify_trust`	`true`	whether trust is checked at all
`remote_manifest_fetch`	`true`	whether reading may fetch over the network
`ocsp_fetch`	`false`	whether revocation is checked over OCSP
`thumbnails`	`false`	embed a thumbnail of the asset and of file-backed ingredients
`thumbnail_size`	1024	longest edge of the thumbnail, in pixels
`thumbnail_format`	smallest	`:jpeg`, `:png` or `:gif`
`thumbnail_quality`	`:medium`	`:low`, `:medium` or `:high`

Settings are global and apply to subsequent calls. Only values you set are sent, so anything left alone keeps c2pa-rs's own default, with one exception: thumbnails is always sent, because this gem's default differs from c2pa-rs's. C2PA.configure with no block resets everything.

Turning verify_trust off means nothing is ever reported as untrusted, which in a library for establishing provenance is rarely what you want. It exists for environments that cannot reach a trust list at all.

Supported file formats

Each format below has a fixture and a signing test in the suite: the file is signed, read back, and asserted to validate.

Format	MIME type
JPEG	`image/jpeg`
PNG	`image/png`
WebP	`image/webp`
TIFF	`image/tiff`
AVIF	`image/avif`
JPEG XL	`image/jxl`
MP4	`video/mp4`
MOV	`video/quicktime`
MP3	`audio/mpeg`
WAV	`audio/wav`
PDF	`application/pdf`	read only, see below

The format is detected automatically from the file extension.

JPEG XL must be in the ISOBMFF container form. A bare codestream has no boxes to hold a manifest, and c2pa-rs rejects it.

PDF is read-only

C2PA.read works on a PDF that carries content credentials, such as one signed by Adobe Acrobat. C2PA.sign does not: c2pa-rs has no PDF writer at any version. get_writer returns None and save_cai_store returns NotImplemented, and upstream closed the request to expose one in December 2025 (contentauth/c2pa-rs#527). Signing a PDF raises C2PA::SigningError with type is unsupported.

Earlier releases of this gem listed PDF as signable. That was never correct.

One limit on what the test suite can show. It proves the PDF handler is active, by reading a PDF with no credentials and getting no JUMBF data found rather than type is unsupported. It cannot prove that a signed PDF returns its manifest, because nothing available can produce one: c2pa-rs cannot write them, and no local tool can either. The code doing the reading is c2pa-rs's own and is tested upstream; what is untested here is only this gem's integration with the success path.

Test fixtures

The media fixtures are generated by test/fixtures/generate.sh from ffmpeg's built-in sources, so they carry no third-party content and no licence obligations. They are deliberately real files rather than placeholders — 160×120 images with actual detail, real audio samples, real video frames, and EXIF metadata on the JPEG — because C2PA writes into container structures that an empty file would not exercise. All ten total 92 KB.

Regenerating them needs ffmpeg, cjxl and exiftool; running the tests does not.

Signing certificates are generated on demand, one chain per key type, so every supported algorithm is covered:

bundle exec rake fixtures:certs

rake test does this for you. The certificates are not committed — they are development material, and regenerating costs a fraction of a second. Ruby's OpenSSL binding is used rather than the openssl command because macOS ships LibreSSL, which cannot generate Ed25519 keys.

How it works

Ruby (C2PA.sign)
    │
    │  native extension (magnus)
    ▼
Rust (C2PA::Native.sign_file)
    │
    │  c2pa-rs Builder, through a shared Context
    ▼
c2pa-rs — embeds signed manifest into the file

The Rust extension (ext/c2pa_native/src/lib.rs) defines C2PA::Native with six methods:

Method	Description
`C2PA::Native.sign_file`	Sign a file and write the result. Takes the manifest JSON, an optional intent, and any ingredient files
`C2PA::Native.sign_buffer`	The same over bytes: a binary string in, the signed binary string out
`C2PA::Native.read_file`	Read and return the manifest JSON
`C2PA::Native.read_buffer`	The same over bytes, with an optional format hint
`C2PA::Native.configure`	Replace the shared c2pa-rs Context with one built from a settings document
`C2PA::Native.sdk_version`	Return the c2pa-rs version string

All four run with the global VM lock released, so other Ruby threads are not blocked while c2pa-rs hashes and signs. The bytes and paths are copied out of Ruby before the lock goes, and the result is turned into a Ruby object after it is back; nothing in between touches the interpreter.

Signing and reading go through one c2pa-rs Context, built once and shared across threads. C2PA.configure replaces it rather than mutating it, so a signing call already in flight keeps the settings it started with.

Input validation (missing files, invalid manifests, unreadable ingredient files, unusable settings) is handled in Ruby before calling into Rust. Errors from the native layer are caught and re-raised as typed C2PA::Error subclasses.

Contributing

See CONTRIBUTING.md. The short version: every test must be shown to fail before it is merged. The suite that shipped with 0.2.1 passed while the gem crashed the Ruby process on TIFF input, so a green run is only worth what its assertions can catch.

License

MIT